Friday's clashes occurred as leaders from the world's 20 biggest developed and emerging economies were to begin a two-day meeting focussing on trade, terrorism, climate change and other key global issues.

Hamburg, a vibrant port city, is a citadel of leftwing radicals and authorities have long been bracing for possible violence on the sidelines of the summit.

The German police officers' union GdP on Friday hit out at anarchist groups called the Black Blocks, accusing them of "hijacking peaceful demonstrations by tens of thousands of people to deliberately attack" police.